Interspecific competition reduces the performance of Atlantic salmon, Salmo salar: implications for restoration programs

Competition from non-native salmonids is potentially impairing efforts to restore Atlantic salmon (Salmo salar) to parts of their historical range. In three separate meta-analyses, we collected 104 effect sizes from 25 published papers to quantify the effect of both native and non-native salmonids on the performance (i.e. behaviour, habitat use, growth and survival) of Atlantic salmon. The presence of other species had negative effects on the performance of Atlantic salmon; in particular, non-native species and brown trout (Salmo trutta), whether native or non-native, had the most negative effects. Contrary to our predictions, the negative effects of other species were not exacerbated in laboratory compared to field studies, and did not increase with total salmonid abundance, or the relative body size of the competitors. However, most studies in our analysis were conducted under laboratory conditions and at densities much higher than found in nature. Thus, a realistic assessment of the potential success of restoration programs when interspecific competitors are present should include more studies conducted under natural conditions.